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Lecture 7: Matter and Energy
Last Time…
forms of energy:
 kinetic
 potential
 radiative
 Law of Conservation of Energy
 energy is not created or destroyed, only
transformed

Temperature and Thermal Energy
Temperature measures the average kinetic
energy of molecules or atoms in a material
(independent of mass)
 Thermal energy or heat measures the total
amount of energy in a material (depends on
mass)

Temperature Scales
Mass as Potential Energy
Einstein showed that mass and energy can
be converted from one to the other
 the amount of energy obtained from
converting a given amount of mass to pure
energy is given by the famous formula

E = m c2
where m is the mass and c is the speed of light,
c = 3 x 108 m/s
Using Einstein’s Equation

How much energy (in Joules) is released by
a 1-megaton H-bomb that converts 0.1 kg of
mass into energy?
E = m c2 = 0.1 kg x (3.0 x 108 m/s)2
= 9 x 1016 kg m2/s2
= 9 x 1016 J
What is Matter?
all matter is made of atoms
 each different kind of atom corresponds to a
different chemical element
 atoms can also be combined to form
molecules

Atoms are not indivisible…
protons (massive, positive electric charge)
 neutrons (massive, neutral electrically)
 electrons (light, negative electric charge)

Atomic mass and number
the identity of an element is determined by
the number of protons (atomic number)
 the atomic mass of an element is the mass
of the protons plus neutrons (electrons have
negligible mass) in units of the proton mass
 an element with the same number of protons
but a different number of neutrons is called
an isotope

Ways to convert mass to energy
nuclear fusion
 two atoms are combined to produce a
heavier atom, releasing energy
 process that stars use to produce light
 nuclear fission
 the nucleus of an atom is split apart,
releasing energy
 used in nuclear power plants

Fission and Fusion
Radioactive Decay
Energy Comparisons
metabolism of a candy bar
1.0 x 106 J
burning 1 liter of oil
1.2 x 107 J
fusion of hydrogen in 1 liter of water
7 x 1013 J
Annual output of the Sun
1034 J
Fusion: the perfect energy source

Suppose that we could build a power plant
fueled by fusion of the Hydrogen in water.
How much water per minute would we need
to meet the entire energy demand of the
USA?
Phases of Matter
 solid
 liquid
 gas
 plasma
Increasing temperature
106 K
plasma
ionization
104 K
molecular dissociation
gas
100 K
liquid
solid
evaporation
103 K
Density
mass density = mass per unit volume
3
3
 units g/cm or kg/m
 density of water is 1 g/cm3
 energy density = energy per unit volume
 units J/ m3

What phase is dominant?
on Earth?
 in our Solar System?
 in our Galaxy?
 in clusters of Galaxies?
 in the Universe?

Coma Cluster